1. Field Of the Invention
The present invention relates to an image processing machine such as a digital copying machine or a facsimile machine, and more particularly to an image processing machine which processes data obtained by reading written or picture documents into binary data, i.e. two values corresponding respectively to black and white.
2. Description of the Related Art
Facsimile or digital copying machines are conventionally known which read image information from documents or pictures with image sensors such as CCDs and which quantize the obtained image information, for example, into binary data and which send the data to recording devices or other machines, collectively "receivers".
In such a conventional machine, such binary processing is performed by using a fixed threshold in the case of a letter document and by using a method, such as the error diffusion method, in which optical density of an original image is retained in the case of a photograph document with or without letters.
U.S. Pat. Nos. 4,969,052 and 4,975,786 and U.S. patent application No. 505,972 (filed Apr. 4, 1990), all made by the present inventor, disclose machines which employ the error diffusion method to perform binary-processing.
In the case of a document having pictures with or without letters, such a conventional machine obtains smooth binary data which retain density of an original image because the binary processing is carried out by a density-retaining method such as the error diffusion method. However, in the case of a document of letters alone, the reproduced letters have rough or notched edge lines because the binary processing is done simply by using a fixed threshold.